1. Field
The invention relates to the nondestructive testing of condensed matter surfaces, and more particularly to the measurement of the work function of the surface of a conductive sample.
2. State of the Art
The work function of the surface of an electronic conductor, solid or liquid, is defined as the minimum amount of work required to move an electron from the interior of the conductor to a point just outside the surface (beyond the image charge region). The work function measured for a particular material will vary if contaminants or coatings are present. Such variations can be used to determine the cleanliness of a surface, the uniformity of thickness of a coating, and other information about the condition of the surface. Thus, work function measurements are employed for nondestructive evaluation of surfaces of various construction parts and materials.
At present, typical methods for determining the absolute work function must be performed in vacuum conditions. However, for industrial purposes it is usually not practical to perform testing in vacuum. Methods which can be used in gas atmospheres generally are useful only to compare the work functions of two surfaces. These methods are based on the fact that there is a contact potential difference (referred to hereinafter as CPD) between the surfaces of two conducting materials that are electrically connected. For purposes of this application, the contact potential difference (CPD) is defined as the difference between the outer potentials (beyond the image charge region) of two electrically connected conducting materials. The outer potentials are themselves related to the work function of the sample surface. This work function depends both on the material of the sample itself, and the condition of the surface which is being examined. Therefore, the CPD between two surfaces reflects the difference in the work functions of their respective sample surfaces.
Typical methods for measuring the CPD involve the creation of a capacitance circuit by electrically connecting the sample and a probe electrode to each other via an external circuit. In this application, such techniques will be designated by the term "capacitance techniques". The probe electrode is positioned just above (but not touching) the sample surface, so that the probe electrode and the sample surface constitute two plates of a capacitor. The capacitance may be varied to cause a current flow which can be detected and correlated to the CPD.
Two such methods will be described in more detail. In the first technique, termed the vibrating capacitor or Kelvin probe technique, the separation between the two capacitor "plates" is varied by vibrating the probe electrode. The changes in plate separation in turn modulate the capacitance and cause an ac current to flow in the external circuit. A variable voltage source in the external circuit is used to apply a voltage to one of the "plates". The voltage is adjusted so that no current flows between the sample and the probe electrode. The voltage required to achieve a null current flow reflects the CPD between the sample and the probe. U.S. Pat. No. 4,072,896 to Bjilmer, U.S. Pat. No. 4,649,336 to Bindner et al, and U.S. Pat. Nos. 4,409,509 and 4,100,442 to Besocke, describe such Kelvin-type probes.
The second capacitance technique which is described is the radioactive probe method. Here, the probe includes a radioactive electrode material which ionizes the gas between the sample and the electrode. The ionization changes the dielectric properties of the capacitor and thereby allows a detectable current to flow. If there is a difference in potential between the reference and the sample, a current will flow across the gap. In the most common application, the variable voltage source is adjusted to change the potential between the capacitor plate lead wires sufficiently to null the current flow. As with the Kelvin probe method, the voltage (potential) required to achieve the null current state reflects the CPD between the two "plates" of the capacitor.
The measurement of CPD in a gas environment by capacitance probes presents problems of reproducibility because adsorption of gas on a surface may cause significant changes in the work function. Such adsorption affects not only the samples being tested, but also the probe. A change in the work function of the probe electrode is virtually indistinguishable from a change in the work function of the sample. Other kinds of surface-gas interactions, as well as changes in weather conditions such as relative humidity, also can strongly influence the measurements made by Kelvin probes and other capacitance probes.
Typical Kelvin probes have used gold or a similar precious metal as the material for the probe electrode in the belief that such metals will not be subject to significant adsorption effects. However, it has been found experimentally that in a gas environment, the work function of even a gold surface drifts at unpredictable rates. For reproducible results, gold probe electrodes must be kept stored under special conditions to minimize adsorption changes. Moreover, gold probes are relatively expensive. In the event that it becomes contaminated with other substances, it cannot be discarded and must instead be carefully cleaned.
Consequently, a need exists for means to more easily and reliably calibrate capacitance measurements of the contact potential difference of a surface in a gas environment. There further exists a need for means to determine the absolute work function of a surface in a gas environment. A need further remains for an inexpensive and stable vibrating capacitor probe.